This application claims the priority of German patent document 100 21 903.9, filed May 5, 2000.
The invention relates to a steering system for a vehicle, which has at least two operating levels, namely a steer-by-wire level and a fall-back level.
A steering system of this kind is fitted with a steering handle, e.g. steering wheel, which can be actuated by a driver of the vehicle and at which an angle of the steering handle can be set. The vehicle fitted with the steering system furthermore has at least one steerable vehicle wheel. Angles of the vehicle wheel are set as a function of actuations of the steering handle with corresponding provided actuating units. The steering system has a control device that allows the steering system to be operated on the steer-by-wire level and on the fall-back level. Normally, the control device operates the steering system on the steer-by-wire level since this generally allows more convenient operation of the steering of the vehicle. The control system continuously monitors the correct operation of the components of the steering system and switches from the steer-by-wire level to the fall-back level in an emergency.
For its steer-by-wire level, such a steering system has an electric and/or electronic coupling device, e.g. a controlled system, which couples the steering handle to the steerable vehicle wheel in the steer-by-wire mode. For the fall-back level, a hydraulic and/or mechanical coupling device, e.g. a so-called hydraulic rod or a steering column that can be engaged, is provided to couple the steering handle to the steerable vehicle wheel in the emergency mode.
With regard to convenience when driving, the advantages of a steer-by-wire steering system are that almost any transfer functions can be achieved because of the absence or inaction of mechanical or hydraulic positive coupling between the steering handle and the steerable vehicle wheel. In this context, these transfer functions can be described with coupling coefficients that depend on operating parameters of the vehicle or of the steering system. For example, a steering ratio can be formed as a function of the vehicle speed and/or the steering angle. This means that at lower vehicle speeds, for example, the steering ratio is different from that at higher speeds. Similarly, the steering ratio at a smaller steering angle can differ from that at larger steering angles. It is furthermore advantageous to form a servo assistance for reducing the forces and torques to be applied at the steering handle by the driver of the vehicle when setting and holding a steering angle as a function of a least one operating parameter of the vehicle, e.g. vehicle speed and steering angle. In this way, it is possible to implement relatively strong servo assistance for maneuvering and relatively weak servo assistance for driving. It is clear that these functional relationships of the coupling coefficients, when dependant upon the operating parameters, can fundamentally have any appropriate characteristic. The coupling coefficients, namely the steering ratio and servo assistance, and the operating parameters, namely the vehicle speed and the steering angle, should not be understood in the sense of a limitation. On the contrary, any coupling coefficients are conceivable that can be formed as a function of any operating parameters are conceivable.
In contrast to these many and various possibilities for a steer-by-wire steering system, such functional relationships between the steering handle and the steerable vehicle wheel can only be implemented to a very limited extent, if at all, with mechanical and/or hydraulic positive coupling between the steering handle and the steerable vehicle wheel. For emergency operation, however, these convenience restrictions can be accepted. In a steering system of the type stated at the outset, there are thus two fundamentally different functional relationships between the steering handle and the steerable vehicle wheel, namely, on the one hand, for the steer-by-wire level and, on the other hand, for the fall-back level. If the control system then switches from the steer-by-wire level to the fall-back level in an emergency, a jump may occur in the functional relationship of a coupling coefficient, e.g. the steering ratio or the servo assistance, and this can have a disadvantageous effect on vehicle safety.
The present invention is concerned with the problem of specifying an embodiment that ensures improved vehicle safety for a steering system of the type stated at the outset.
The invention is based on the general idea of matching the fundamentally different functional relationships between the steering handle and the steerable vehicle wheel in such a way on the steer-by-wire level, on the one hand, and on the fall-back level, on the other hand, that the coupling coefficients are so close together from a certain limiting value of the operating parameter onwards that a switch between the operating levels of the steering system produces only a small, tolerable, jump or none at all in the dependence between the coupling coefficient and the operating parameter. A difference between the coupling coefficient of the steer-by-wire level and the coupling coefficient of the fall-back level below the stated limiting value for the operating parameter can normally be relatively large, while, according to the invention, this difference is relatively small above the stated limiting value. The magnitude of this difference is defined by a predetermined limiting difference. In this way, it is possible to avoid dangerous driving situations when switching to the fall-back level, thereby increasing vehicle safety.
For example, a steering ratio between the angle of the steering handle and the angle of the vehicle wheel and/or a servo assistance for reducing the forces or torques to be applied at the steering wheel by the driver of the vehicle when setting and holding a steering angle can each form a coupling coefficient which is formed as a function of at least one operating parameter of the vehicle or of the steering system. Functional relationships with the vehicle speed and/or the vehicle acceleration and/or the steering angle are preferred as operating parameters of the vehicle. In this way, it is possible, for example, to represent the steering ratio as a function of the vehicle speed.
The limiting difference prevailing between the coupling coefficients above the limiting value is expediently chosen to be so small that, although a jump in the coupling coefficient when switching between the steer-by-wire level and the fall-back level does cause a disturbance that can be felt by the driver of the vehicle, e.g. at the steering handle, this is within a tolerance range that is permissible for normal driving of the vehicle. Even during normal driving operation, disturbances may occur when, for example, the vehicle runs over an irregularity in the road surface, although this generally leads to a driving situation that can be managed by the driver of the vehicle. In this variant, the extent of the disturbance caused by the jump in the coupling coefficient will therefore generally be such that it can be managed by the driver of the vehicle. However, an embodiment in which the limiting difference is approximately or precisely zero is preferred. The switch from the steer-by-wire level to the fall-back level then takes place without a jump in the characteristic of the coupling coefficient and therefore no disturbance can be felt at the steering handle. This further reduces the risk of a critical driving situation.
In a preferred development of the steering system according to the invention, the control system can switch from the steer-by-wire level to the fall-back level at the limiting value of the operating parameter, irrespective of an emergency. This ensures that a deliberate switch to the fall-back level takes place when no dangerous situation can arise on the basis of the prevailing operating states and the existing operating parameters. If an emergency then occurs above the limiting value in the case of a critical operating parameter, the steering system is already being operated on its fall-back level, with the result that no switching process is required, thereby avoiding a critical jump in the characteristic of the respective coupling coefficient. The steering system can, for example, be designed in such a way that the fall-back level is always adopted from a certain vehicle speed onwards.
In another embodiment, there is a plurality of limiting values of increasing magnitude, which are assigned limiting differences of decreasing magnitude. In this way, it is possible, for example, to implement an embodiment in which the coupling coefficients of the steer-by-wire level and of the fall-back level have a relatively large difference at a relatively low speed and have a relatively small difference at a moderate vehicle speed, and are approximately equal at higher vehicle speeds.
In particular, the limiting value of the operating parameter can be used to divide maneuvering operation from driving operation, resulting in particularly convenient steering behaviour of the vehicle.
According to another embodiment, the steering system can be fitted with means for carrying out steering interventions, these means producing steering interventions to stabilize the vehicle independently of the driver of the vehicle but in a manner dependent on driving situations. In a steering system of this kind, these steering interventions take place in such a way, from the limiting value of the operating parameter onwards, that the limiting difference is maintained between the coupling coefficients. This ensures that an emergency and thus the associated switch to the fall-back level does not produce an uncontrollable driving situation even if the switch occurs precisely at a time when a steering intervention is being performed to stabilize the vehicle.
It is self-evident that the features mentioned above and those that will be explained below can be employed not only in the respectively indicated combination but also in different combinations or in isolation without departing from the scope of the present invention.
A preferred exemplary embodiment of the invention is illustrated in the drawings and is explained in greater detail in the following description.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.